Great... Now I have images of Severus scratching somewhere less dignified.
Though the image is made truly comic by trying to imagine him scratching his crotch through layers of thick, dark robes.
Unless of course you just meant dandruff or something like that...
Or maybe he was just massaging his sore bum or (since this is YOUR fic) adjusting Merlin knows what that is stuck up his butt and it seemed like undignified scratching.